The present invention relates to an exhaust gas recirculation control device of an engine.
Conventionally, it has been known to provide a high-pressure exhaust gas recirculation (EGR) passage and a low-pressure EGR passage to recirculate a part of exhaust gas from an engine, from an exhaust passage to an intake passage. For example, JP2007-315371A discloses such a configuration. The high-pressure EGR passage connects a part of the exhaust passage upstream of a turbine of an exhaust turbocharger disposed therein, to a part of the intake passage downstream of a compressor of the exhaust turbocharger. Moreover, the low-pressure EGR passage connects a part of the exhaust passage downstream of the turbine to a part of the intake passage upstream of the compressor. A low-pressure EGR valve for changing a cross-sectional area of the low-pressure EGR passage is disposed in the low-pressure EGR passage, and a high-pressure EGR valve for changing a cross-sectional area of the high-pressure EGR passage is disposed in the high-pressure EGR passage. Normally, an opening of the low-pressure EGR valve is controlled so that an amount of exhaust gas of the engine recirculated by the low-pressure EGR passage becomes a low-pressure EGR target recirculation amount set in advance, and an opening of the high-pressure EGR valve is controlled so that an amount of exhaust gas recirculated by the high-pressure EGR passage becomes a high-pressure EGR target recirculation amount set in advance.
In JP2007-315371A, an actual exhaust gas recirculation amount is calculated by subtracting an intake air amount (fresh air amount) from a total amount of intake gas introduced into the engine. Moreover, JP2008-169724A discloses a configuration in which a high-pressure EGR passage is provided (a low-pressure EGR passage is not provided) and a high-pressure EGR valve is provided therein, and an actual amount of exhaust gas recirculated in high-pressure by the high-pressure EGR passage (actual high-pressure recirculation amount) is calculated based on a pressure difference on the upstream side and the downstream side of the high-pressure EGR valve and also an opening of the high-pressure EGR valve.
Meanwhile, in a case of performing a failure diagnosis of the low-pressure EGR valve, when the exhaust gas is recirculated only by the low-pressure EGR passage while the engine is in a normal operation, an actual low-pressure recirculation amount calculated by subtracting the fresh air amount from the total amount of intake gas introduced into the engine as JP2007-315371A is compared with the low-pressure EGR target recirculation amount. If the actual low-pressure recirculation amount significantly deviates from the low-pressure EGR target recirculation amount, the actual low-pressure recirculation amount is determined to be abnormal, and in this case, the low-pressure EGR valve can be determined as broken.
On the other hand, in a case of performing a failure diagnosis of the high-pressure EGR valve, while the engine is in the normal operation, the actual high-pressure recirculation amount calculated based on the pressure difference on the upstream side and the downstream side of the high-pressure EGR valve and also an actual opening of the high-pressure EGR valve as JP2008-169724A is compared with the high-pressure EGR target recirculation amount. If the actual high-pressure recirculation amount significantly deviates from the high-pressure EGR target recirculation amount, the actual high-pressure recirculation amount is determined to be abnormal, and in this case, the high-pressure EGR valve can be determined as broken.
However, in the case of performing the failure diagnosis of the high-pressure EGR valve, since a pressure of the high-pressure EGR valve on the exhaust passage side varies greatly due to exhaust gas pulsation, it is difficult to accurately obtain the pressure difference on the upstream side and the downstream side of the high-pressure EGR valve, and thus, the reliability of the determination result of whether the actual high-pressure recirculation amount is normal/abnormal, in other words, the reliability of the failure diagnosis of the high-pressure EGR valve, is low. On the other hand, on the intake side, since the pulsation is not strong, the total intake gas amount and the fresh air amount can be obtained comparatively accurately, and the reliability of the determination result of whether the actual low-pressure recirculation amount is normal/abnormal, in other words, the reliability of the failure diagnosis of the low-pressure EGR valve, is high.